One of the currently used electron guns is a multifocusing type electron gun in which a unipotential electrostatic lens and a bipotential electrostatic lens are combined as described in Japanese Patent Publication No. Sho-62-58102 and as schematically illustrated in FIG. 1. Such an electron gun has an improved characteristics in spherical aberration and astigmatism over the electron gun having a single lens.
As shown in FIG. 1, in such an electron gun, a unipotential electrostatic lens PFL, which comprises prefocus electrodes G3, G4, and G5a, is disposed between a triode which comprises cathodes K, a control electrode G1, and a screen electrode G2, and a main lens MFL.
In such an electron gun, further improvements of performance are still required in focusing characteristics on the screen of the cathode ray tube. Since the aforesaid conventional electron gun includes a unipotential prefocus lens and a bipotential main focus lens, it is likely that beam spots on the screen might be expanded due to strong focusing forces of the above mentioned two focus lenses.
In overcoming the above problem, there is a method such that focusing forces of the prefocus lens are properly weakened through a proper structural modification, so that the incident angle of the electron beam entering the final main focus lens is expanded.
The electron gun having the aforesaid uni-bipotential electrostatic lens is further constituted such that outer electron beams are made to converge into a middle electron beam by an asymmetrical main lens. The asymmetrical lens is formed by outer beam passing holes of a final focusing electrode G5b and an anode G6. The outer beam passing hole of anode G6 is disposed outwardly eccentrically relative to the outer beam passing hole of final focusing electrode G5b.
In such an electron gun, the electron beams are converged at the final main focus lens which is disposed adjacent the screen of the cathode ray tube. Therefore, it is possible that an astigmatism occurs along the periphery of the screen.